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water separation
Progress in Organic Coatings 115 (2018) 172–180
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Superhydrophobic mGO/PDMS hybrid coating on polyester fabric for oil/ water separation ⁎
Xiaofeng Liao, Hongqiang Li , Lin Zhang, Xiaojing Su, Xuejun Lai, Xingrong Zeng
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College of Materials Science and Engineering, Key Lab of Guangdong Province for High Property and Functional Polymer Materials, South China University of Technology, Guangzhou 510640, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene oxide Hybrid coating Superhydrophobic fabric Oil/water separation Reusability
With the environment pollution deteriorating by industrial waste water and oil leak, superhydrophobic materials for oil/water separation have attracted more attentions and become a worldwide research hotspot. Herein, we propose a facile dipping-UV curing approach to fabricating superhydrophobic hybrid coating on polyester fabric for oil/water separation by utilizing (3-mercaptopropyl) trimethoxysilane modified graphene oxide (mGO) and vinyl-terminated polydimethylsiloxane (V-PDMS). With mass ratio of mGO to V-PDMS increasing, the roughness of the fabricated mGO/PDMS hybrid coating on polyester fabric correspondingly increased and the water contact angle (WCA) reached up to 157°, demonstrating excellent water repellency. Importantly, due to the formation of crosslinked PDMS network with mGO as crosslinking points, the fabric exhibited good thermal stability and chemical resistance. Furthermore, the superhydrophobic fabric also possessed high oil/water separation efficiency at 99.8%, and still kept at 98.4% even after 15 separation cycles. Our findings stand out as a new tool to fabricate superhydrophobic materials and coatings with graphene and its derivatives for oil/water separation, showing great potential in practical applications such as oil spill accidents and industrial sewage emissions.
1. Introduction Inspired by lotus leaves [1], rose petals [2], water strides [3], and other various plants and insects [4–7] in nature, superhydrophobic materials with water contact angle (WCA) above 150° and sliding angle (SA) below 10° [8–10] have aroused much attention for their potential applications in self-cleaning [11,12], anti-icing [13,14], anti-fouling [15], drag reduction [16] and oil/water separation [17,18]. In the past two decades, with industrial waste water discharge and oil spill accidents increasing [19,20], the environment and ecology have been suffering from serious contamination. Therefore, to fabricate superhydrophobic materials for efficient oil/water separation is an important research direction worldwide. Generally, hierarchical micro/nanostructures and low-surface-energy substance are considered to be the two key factors for constructing superhydrophobic materials [21–23]. Based on this principle, several representative methods have been developed to prepare superhydrophobic materials, including lithography [5,24], self-assembly [25,26], electrospinning [27,28], chemical vapor deposition (CVD) [29,30], sol-gel process [31,32], etching [33–35] and templating [36]. However, the harsh operating conditions, special equipment and timeconsuming process limit their large-scale production and application.
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Graphene and its derivatives have been favorably used in versatile composite materials to improve thermal, mechanical, chemical and electrical performances [37–39]. In recent years, due to the intriguing hydrophobicity, graphene has been consecutively proposed to fabricate superhydrophobic materials [40–42]. Javad et al. [43] fabricated a thermally exfoliated graphene film with controllable superhydrophobicity-superhydrophilicity by adjusting the relative proportion of acetone and water. After solvothermal reduction of the mixed dispersion of graphene oxide and PVDF, Li et al. [44] prepared a superhydrophobic and superoleophilic graphene/polymer aerogels with high absorption capacity for oils and organic solvents. By taking advantages of the low polarizability of perfluorinated carbons and the intrinsic conductive nature of graphene nanoribbons, Wang et al. [45] also prepared a perfluorododecylated graphene nanoribbon film with excellent anti-icing and deicing properties. In Li’s study [46], graphene oxide was firstly treated with spark plasma sintering (SPS) and then reduced at 1050 °C for 1 min, the obtained SPS-reduced graphene oxide demonstrated a WCA of 153° and possessed an impressive bacterial antifouling and inactivation effect against Escherichia coli. From the above literatures, it is obvious that the incorporation of toxic fluorinecontaining chemicals and high-temperature treatment with expensive equipment are usually necessary for the fabrication of graphene-based
Corresponding author at: College of Materials Science and Engineering, South China University of Technology, No 381, Wushan Road, Tianhe District, Guangzhou 510640, China. E-mail addresses: [email protected] (H. Li), [email protected] (X. Zeng).
https://doi.org/10.1016/j.porgcoat.2017.12.001 Received 29 September 2017; Received in revised form 16 November 2017; Accepted 1 December 2017 0300-9440/ © 2017 Elsevier B.V. All rights reserved.
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and dried at 80 °C for 1 h. Next, a predetermined amount of mGO (e.g. the mass ratios of mGO to V-PDMS was varied at 0, 0.1, 0.25 and 0.5, respectively) was added into 10 g of acetone solution containing 0.3 g of V-PDMS, 0.03 g of TMPTA and 0.01 g of Darocur 1173, and sonicated for 30 min to form a homogeneous solution. Subsequently, the dried fabric was dipped into the acetone solution and sonicated for 30 min in the ice water bath to keep the temperature constant, then taken out and exposed under UV light (INTELLI-RAY 400, Uvitron International, Inc., U.S.A.) for 2 min with a distance of 15 cm between the sample and the center of UV light lamp. Thus, the superhydrophobic mGO/PDMS hybrid coating on fabric was obtained.
superhydrophobic materials and surfaces. Recently, our group [47] prepared a thiolated graphene-based superhydrophobic polyurethane sponge with high absorption selectivity and oil/water separation efficiency, and the thiolation process of graphene oxide to thiolated graphene was simple and low-cost. However, the adhesion between thiolated graphene and sponge skeleton still needs to be further strengthened in practical oil/water separation. Herein, we propose a facile approach to fabricating superhydrophobic hybrid coating on polyester fabric for oil/water separation with graphene material. Firstly, graphene oxide (GO) was modified with (3-mercaptopropyl) trimethoxysilane (MPTMS) to achieve hydrophobic mGO with thiol groups. Secondly, the pristine polyester fabric was immersed into the acetone solution containing mGO, vinyl-terminated polydimethylsiloxane (V-PDMS), trimethylolpropane triacrylate (TMPTA) and photoinitiator Darocur 1173, and then exposed under UV light to form crosslinked mGO/PDMS hybrid coating on fabric. The asfabricated coating on polyester fabric exhibited rough morphology and superhydrophobicity. Importantly, the superhydrophobic fabric possessed high oil/water separation efficiency at 99.8% and good reusability. Our method is simple, efficient and cost-effective, and has great potential in the treatment of oil spill accidents and industrial sewage emissions.
2.4. Stability evaluation of superhydrophobic mGO/PDMS hybrid coating on polyester fabric The thermal stability of the superhydrophobic mGO/PDMS hybrid coating on fabric was evaluated by measuring the water contact angle (WCA) after being heated in the range of 30–150 °C with an increasing interval of 30 °C for every 6 h. To comprehensively evaluate the chemical stability, the superhydrophobic fabrics were separately immersed into various solutions including water, toluene, hexane, NaCl solution (1 mol/L), HCl solution (pH = 1) and NaOH solution (pH = 13) for different time, and then rinsed with ethanol and dried at 80 °C for WCA test. In addition, the mechanical stability was also studied via the abrasion test. The 800 mesh sandpaper was acted as the abrasion material, and the superhydrophobic fabric sample was pressed by a weight of 200 g in one direction, and the dragging distance was 15 cm for one cycle.
2. Experiment section 2.1. Materials Graphite powder (325 mesh), sodium nitrate (NaNO3), (3-mercaptopropyl) trimethoxysilane (MPTMS), trimethylolpropane triacrylate (TMPTA, 85%), 2-hydroxy-2-methylproplophenone (Darocur 1173), sodium chloride, hexane, ethanol and oil red O were all provided by Aladdin reagent Co., Ltd., China. Hydrogen peroxide (H2O2, 30%) was bought from Chinasu Specialty Products Co., Ltd., China. Potassium permanganate (KMnO4), sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl, 37%), sodium hydroxide, acetone, toluene, petroleum ether, dichloromethane and trichloromethane were all supplied by Guangzhou Chemical Reagent Factory, China. Vinyl-terminated polydimethylsiloxane (V-PDMS, average MW = 6000) was purchased from Meryer (Shanghai) Chemical Technology Co., Ltd. Methylene blue was obtained from Tianjin Tianxin fine chemical development center (China). Polyester fabric (plain weave fabric, 120 g/m2) was bought from local market. All chemicals were used as received without further purification, and deionized water was used for all the experiments and tests.
2.5. Oil/water separation Oil/water separation test was carried out with a simple homemade filter device composed of two glass tubes and two metal clips. The fabricated superhydrophobic fabric acting as filter membrane was sandwiched between the two glass tubes, and then firmly fixed using metal clips. When oil/water mixture was poured into the upper tube, water was impeded in the tube owing to the water repellency of the fabric, while oil automatically penetrated through the fabric and was collected in the container under the glass tube. Several different kinds of light oil and heavy oil including hexane, toluene, trichloromethane, dichloromethane and petroleum ether were selected for separation test. The oil/water separation efficiency was calculated according to the following Eq. (1):
S eparationefficiency = 2.2. Preparation of MPTMS modified graphene oxide (mGO)
m1 × 100% m0
(1)
where m0 and m1 represented the mass of initial oil and collected oil, respectively.
GO was synthesized from graphite powder according to the modified Hummers’ method [47,48] (see Supporting Information). To achieve the modification of GO, 5 mL of MPTMS and 100 mL of deionized water were firstly added to a 500-mL three-neck flask equipped with a mechanical stirrer and a condenser pipe, and then the pH value was adjusted to 4–5 by adding HCl solution (1 mol/L). After that, 60 mL of GO aqueous solution (3.5 mg/mL) was added under stirring. Subsequently, the mixture was heated to 90 °C and kept for 6 h. Finally, the cooled mixture was filtrated, washed with plenty of deionized water and ethanol separately for three times, then dried at 50 °C for 12 h, and the black mGO was obtained for the next use.
2.6. Characterizations The surface morphologies of the mGO and superhydrophoic fabric were observed on an EVO18 scanning electron microscope (SEM, Carl Zeiss Jena, Germany) at an acceleration voltage of 10.0 kV under high vacuum condition. Fourier transform infrared (FT-IR) spectroscopy was carried out on a Bruker Tensor 27 spectrometer (Bruker Optics, Germany) in the range of 4000–400 cm−1 with a resolution of 4 cm−1 and scanning times of 32, and samples were prepared by the potassium bromide (KBr) tableting method. Chemical compositions of GO and mGO were performed on a X-ray photoelectron spectroscope (XPS, Kratos Axis Ulra DLD, UK) equipped with Al Kα monochromatic X-ray source and three electron take-off angles (30°, 60° and 90°). The surface microstructure and roughness of the pristine and fabricated fabrics were analyzed with atomic force microscopy (AFM, Bruker Multimode 8, USA) in tapping mode with a scanning rate of 0.977 Hz in 3 μm × 3 μm scale. The grafting ratio of MPTMS on GO was
2.3. Fabrication of superhydrophobic mGO/PDMS hybrid coating on polyester fabric The schematic illustration for fabricating superhydrophobic mGO/ PDMS hybrid coating on fabric is presented in Fig. 1. Typically, the pristine polyester fabric with suitable size (25 mm × 25 mm) was separately cleaned with ethanol and deionized water under sonication, 173
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Fig. 1. Schematic illustration for fabricating superhydrophobic mGO/PDMS hybrid coating on fabric.
vibration and alkoxy CeO stretching, respectively [50]. After modification, the intensity of the broad peak at 3392 cm−1 significantly decreased, and a new weak band at 2553 cm−1 corresponding to S-H vibration of MPTMS appeared, confirming the successful grafting of MPTMS on GO [51]. The XPS spectra of GO and mGO are shown in Fig. 3b. Notably, there were only C1s and O1s peaks in the XPS survey spectrum of GO. Differently, the new Si2s, Si2p, S2s and S2p peaks were observed in that of mGO. Additionally, the intensity of O1 s peak significantly decreased and the C1s peak slightly increased, which led to a high C/O atom ratio of mGO at 11.56. The high-resolution C1s corelevel spectra of GO and mGO are displayed in Fig. 3c and d, respectively. It can be seen that the typical XPS C1s spectrum of GO was divided into five peaks at 284.7, 285.6, 286.1, 288.2 and 289.1 ev, attributing to C]C, CeC, CeO, C]O and OeC]O, respectively [51,52]. Comparatively, in the C1s spectrum of mGO, the peak intensity of CeO, C]O and OeC]O dramatically decreased, and the new characteristic peaks of Si-C at 283.7 ev and C-S at 285.9 ev were observed, amply demonstrating that MPTMS had been grafted on GO nanosheets [47,51]. The grafting ratio of MPTMS on GO was also determined by TGA (see Fig. S1) and calculated to be approximately 20.8%.
determined with a TG209 thermogravimetric analyzer (Netzsch, Germany) heating from room temperature to 700 °C at 10 °C/min under nitrogen atmosphere. WCAs were measured with a contact angle meter (DSA100, Germany) equipped with a video capture using about 5 μL of water drops as probe at room temperature, and at least five different positions were selected for measurement to calculate the average value.
3. Results and discussion 3.1. Preparation and structure characterizations of mGO Generally, due to the existence of polar carboxyl, hydroxyl, carbonyl and epoxy groups, hydrophilic GO is not considered to be appropriate for preparing superhydrophobic materials. However, by means of the polar groups especially hydroxyl and carboxyl groups as reactive points, GO can be easily modified to achieve hydrophobicity. In our work, GO was modified with MPTMS to prepare hydrophobic mGO by utilizing the hydrolysis of MPTMS to form −Si-OH groups and the subsequent condensation with hydroxyl groups on GO. The photograph of GO and mGO in water with the same mass concentration of 20% is presented in Fig. 2a. It was notable that GO was well dispersed in water, and the aqueous solution appeared homogeneous brownyellow color. In contrast, the black mGO was unable to disperse in water, and precipitated at the bottom of the bottle. Correspondingly, the WCA of GO was only 31°, and that of mGO reached to 128° as shown in Fig. 2b. The obvious changes in color and WCA can be attributed to the removal of most polar oxygen-containing groups [49]. The SEM images of GO and mGO are shown in Fig. 2c and d, respectively. Clearly, GO exhibited a lamellar structure with some wrinkled and scrolled shapes, and mGO possessed the similar nanosheets while accompanying with some more dense and folded regions. As can be seen from FT-IR spectra in Fig. 3a, the most characteristic peaks of GO located at 3392, 1718, 1622, 1398, and 1082 cm−1 were ascribed to OeH stretching, C]O stretching, C]C stretching, OeH
3.2. Fabrication of superhydrophobic mGO/PDMS hybrid coating on polyester fabric In our work, mGO was not only utilized to construct the roughness of the hybrid coating on the fibers, but also acted as crosslinking points for V-PDMS to form three-dimension crosslinked network structure through the thiol-ene reaction between −SH groups of mGO and terminated eC]C groups of V-PDMS under UV light. Additionally, the residual polar groups on mGO were also beneficial for its adhesion with the polar polyester fibers. By a simple dipping-UV curing method, the superhydrophobic mGO/PDMS hybrid coating on fabric was easily fabricated with mGO, V-PDMS, TMPTA and Darocur 1173. As shown in FT-IR spectrum of the fabric before UV curing (see Fig. S2), the peaks at 174
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Fig. 2. (a) Photograph of GO (left) and mGO (right) in water; (b) WCA images of GO at 31° and mGO at 128°; SEM images of (c) GO and (d) mGO.
1298 cm−1 and 798 cm−1 of Si-C and 1089 cm−1 of Si-O-Si verified the existence of V-PDMS, and the representative peaks locating at 2962 cm−1 and 2900 cm−1 were attributed to the stretching vibrations of −CH3 and −CH2- on mGO and V-PDMS chains [53]. In addition, the stretching vibrations of C]O and OeH assigning to the residual carboxy groups were also observed at 1735 cm−1 and 1404 cm−1, respectively. After UV curing, the weak peaks at 2561 cm−1 of S-H groups of mGO and 1627 cm−1 of eC]C groups of V-PDMS disappeared, demonstrating the successful thiol-ene reaction between −SH groups of mGO and terminated eC]C groups of V-PDMS.
It is well known that the construction of hierarchical micro/nanostructures is crucial for the achievement of superhydrophobic materials. The SEM images and the corresponding WCAs of the superhydrophobic fabrics fabricated with different mass ratios of mGO to VPDMS are illustrated in Fig. 4. Obviously, the pristine polyester fabric showed smooth surfaces and water droplets were gradually absorbed by the hydrophilic fabric (see Fig. S3). The fabric surface only covered with PDMS layer was quite smooth and the WCA was 135° (Fig. 4a). With the mass ratio of mGO to V-PDMS at 0.1, some protrusions were observed on the surface of the fibers and the WCA attained 143° Fig. 3. (a) FT-IR spectra of GO and mGO; (b) XPS survey scans of GO and mGO; (c) C1s XPS spectrum of GO; (d) C1s XPS spectrum of mGO.
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Fig. 4. SEM images of mGO/PDMS hybrid coating on polyester fabrics with different mass ratios of mGO to V-PDMS at (a) 0; (b) 0.1; (c) 0.25 and (d) 0.5, respectively.
interpreted by Cassie-Baxter model [54]. Furthermore, as shown in Fig. 6c, when a continuous jet of water was sprayed on the fabricated fabric, it easily bounced off the surface, further illustrating the excellent water repellency.
(Fig. 4b). When the mass ratios further increasing to 0.25 and 0.5, the hierarchical micro/nano structure of the fiber surface significantly increased, and the WCAs reached to 157° and 153°, respectively (Fig. 4c and d). However, it was worth noting that the WCA of the fabric fabricated with the mass ratio at 0.5 was slightly lower than that at 0.25, which was due to the formation of the enlarged protrusions by the serious agglomeration of mGO nanosheets. Thus, the fabric fabricated with the mass ratio of mGO to V-PDMS at 0.25 was chosen for the following tests. To further investigate the surface roughness of the mGO/PDMS hybrid coating on fabric, AFM was performed and the morphology images of the pristine fabric and the fabricated superhydrophobic fabric are presented in Fig. 5. As shown in Fig. 5a, the surface of the pristine fabric was quite smooth, and the root-mean-square roughness (RMS) was calculated to be only 1.03 nm. In comparison, the superhydrophobic fabric in Fig. 5b obviously exhibited a higher hierarchical roughness with the obvious increase of RMS to 12.4 nm, which was in good consistent with the above SEM and WCA results. Fig. 6a shows the photograph of water and hexane droplets on the pristine fabric and the fabricated fabric, respectively. It was clear that water droplets quickly permeated into the pristine fabric only in several seconds. Differently, on the fabricated fabric, water droplets could keep their sphere shape all the time and even stand on the oil-contaminated area, exhibiting the superhydrophobicity and superoleophilicity. When pressed into water by an external force, the fabricated fabric appeared a mirror-like surface in Fig. 6b, as a result of a uniform air layer existing between the coating layer on fabric and water, which could be
3.3. Stability of superhydrophobic mGO/PDMS hybrid coating on polyester fabric To evaluate the stability, the superhydrophobic fabric was heated subsequently to 30 °C, 60 °C, 90 °C, 120 °C and 150 °C for 6 h, and also separately immersed in water, hexane, toluene, NaCl solution, strong acid and alkali solution for different time to compare the changes of the WCAs. As shown in Fig. 7a, the WCAs of the superhydrophobic fabric had almost no change after heating for 6 h at different temperatures, showing the strong thermal stability. Furthermore, the WCAs of the fabricated fabrics still remained above 150° after being separately immersed into water, hexane, toluene and NaCl solution for 48 h as illustrated in Fig. 7b, and the corresponding photographs of water droplets standing on the treated fabric clearly revealed the good chemical stability of the superhydrophobic fabric (see Fig. S4). Meanwhile, it was notable that the WCAs of the fabricated fabric had no obvious decrease even after being immersed into strong acid solution (pH = 1) for 48 h and alkali solution (pH = 13) for 12 h, respectively. Additionally, the surface morphologies changed little as shown in Fig. 7c and d. The abrasion test was also carried out to evaluate the mechanical stability of the superhydrophobic fabric (see Fig. S5a). After 100 abrasion cycles, the WCA decreased a little, while the water droplets still remained Fig. 5. AFM images of the surface of (a) pristine fabric and (b) superhydrophobic fabric.
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Fig. 6. (a) Photograph of water and hexane droplets on the pristine fabric (left) and the fabricated fabric (right); (b) Photograph of the fabricated fabric immersed into water by an external force; (c) Photograph of a continuous jet of water sprayed on the fabricated fabric.
spherical on the seriously worn-out areas (Fig. S5b). It was mainly attributed to the formation of the crosslinked PDMS network with mGO as crosslinking points.
3.4. Oil/water separation of the superhydrophobic polyester fabric With the increasing industrial sewage and deteriorating marine environment, superhydrophobic material has been considered to be a desirable candidate for oil/water separation. As shown in Fig. 8a and b, Fig. 7. (a) WCAs of the fabricated fabrics heated at different temperatures for 6 h; (b) WCAs of the fabricated fabrics immersed in various solutions; SEM images of the fabricated fabrics after being immersed in (c) HCl solution (pH = 1) for 48 h and (d) NaOH solution (pH = 13) for 12 h.
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Fig. 8. Photographs of the superhydrophobic polyester fabrics selectively absorbing (a) hexane and (b) trichloromethane droplets from water.
separation efficiencies were all above 90% (Fig. 10a). Furthermore, the superhydrophobic fabric exhibited excellent reusability and still remained high separation efficiency at 98.4% after 15 separation cycles (Fig. 10b).
when several dyed droplets of hexane and trichloromethane as the representatives of light oil and heavy oil were dropped into two beakers with water, they quickly spread on water surface and the bottom of the beaker, respectively. Notably, it was very convenient to completely separate the oil from water with the superhydrophobic fabric just in several seconds (see Video S1 and S2). A simple oil/water separation device was set up to further study the massive amount of oil/water mixture. It was composed of two glass tubes, two metal clips and a piece of superhydrophobic polyester fabric. As can be seen from the separation process of oil/water mixture shown in Fig. 9, when trichloromethane/water mixture was poured into the upper glass tube, the red trichloromethane easily penetrated through the fabric and quickly flowed into the container along with the glass tube under the force of gravity, while water was impeded in the upper glass tube owing to the superhydrophobicity of the fabric (see Video S3), and the separation efficiency reached 99.8%. Additionally, several kinds of oils including hexane, toluene, dichloromethane and petroleum ether were also selected as representatives to comprehensively evaluate the oil/water separation ability of the superhydrophobic fabric, and the
4. Conclusions To summarize, we demonstrate a facile dipping-UV curing approach to fabricate superhydrophobic mGO/PDMS hybrid coating on polyester fabric for oil/water separation. With the mass ratio of mGO to V-PDMS at 0.25, the WCA of the fabricated fabric reached 157°, and the corresponding fiber surface appeared micro/nano structure and hierarchical roughness. Due to the formation of the crosslinked PDMS network with mGO as crosslinking points, the superhydrophobic polyester fabric also exhibited excellent thermal stability and chemical stability. Importantly, the fabric could effectively separate oil/water mixture, and possessed high separation efficiency and excellent reusability. The fabrication method is simple, efficient and available for large-scale production, and has the great potential in the treatment of oil spill
Fig. 9. Snapshots for the separation process of trichloromethane/water mixture with the superhydrophobic fabric.
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Fig. 10. (a) Separation efficiencies of the superhydrophobic fabrics for different oil/water mixtures and (b) Separation efficiency of the superhydrophobic fabrics for trichloromethane/water mixture after different separation cycles.
accidents and industrial sewage emissions. Our findings conceivably stand out as a new tool to prepare graphene-based superhydrophobic materials.
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Acknowledgements The work was financially supported by the National Natural Science Foundation of China (51403067) and the Pear River S&T Nova Program of Guangzhou (201710010062).
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Appendix A. Supplementary data [23]
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.porgcoat.2017.12.001.
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Superhydrophobic mGO/PDMS hybrid coating on polyester fabric for oil/ water separation ⁎
Xiaofeng Liao, Hongqiang Li , Lin Zhang, Xiaojing Su, Xuejun Lai, Xingrong Zeng
T
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College of Materials Science and Engineering, Key Lab of Guangdong Province for High Property and Functional Polymer Materials, South China University of Technology, Guangzhou 510640, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene oxide Hybrid coating Superhydrophobic fabric Oil/water separation Reusability
With the environment pollution deteriorating by industrial waste water and oil leak, superhydrophobic materials for oil/water separation have attracted more attentions and become a worldwide research hotspot. Herein, we propose a facile dipping-UV curing approach to fabricating superhydrophobic hybrid coating on polyester fabric for oil/water separation by utilizing (3-mercaptopropyl) trimethoxysilane modified graphene oxide (mGO) and vinyl-terminated polydimethylsiloxane (V-PDMS). With mass ratio of mGO to V-PDMS increasing, the roughness of the fabricated mGO/PDMS hybrid coating on polyester fabric correspondingly increased and the water contact angle (WCA) reached up to 157°, demonstrating excellent water repellency. Importantly, due to the formation of crosslinked PDMS network with mGO as crosslinking points, the fabric exhibited good thermal stability and chemical resistance. Furthermore, the superhydrophobic fabric also possessed high oil/water separation efficiency at 99.8%, and still kept at 98.4% even after 15 separation cycles. Our findings stand out as a new tool to fabricate superhydrophobic materials and coatings with graphene and its derivatives for oil/water separation, showing great potential in practical applications such as oil spill accidents and industrial sewage emissions.
1. Introduction Inspired by lotus leaves [1], rose petals [2], water strides [3], and other various plants and insects [4–7] in nature, superhydrophobic materials with water contact angle (WCA) above 150° and sliding angle (SA) below 10° [8–10] have aroused much attention for their potential applications in self-cleaning [11,12], anti-icing [13,14], anti-fouling [15], drag reduction [16] and oil/water separation [17,18]. In the past two decades, with industrial waste water discharge and oil spill accidents increasing [19,20], the environment and ecology have been suffering from serious contamination. Therefore, to fabricate superhydrophobic materials for efficient oil/water separation is an important research direction worldwide. Generally, hierarchical micro/nanostructures and low-surface-energy substance are considered to be the two key factors for constructing superhydrophobic materials [21–23]. Based on this principle, several representative methods have been developed to prepare superhydrophobic materials, including lithography [5,24], self-assembly [25,26], electrospinning [27,28], chemical vapor deposition (CVD) [29,30], sol-gel process [31,32], etching [33–35] and templating [36]. However, the harsh operating conditions, special equipment and timeconsuming process limit their large-scale production and application.
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Graphene and its derivatives have been favorably used in versatile composite materials to improve thermal, mechanical, chemical and electrical performances [37–39]. In recent years, due to the intriguing hydrophobicity, graphene has been consecutively proposed to fabricate superhydrophobic materials [40–42]. Javad et al. [43] fabricated a thermally exfoliated graphene film with controllable superhydrophobicity-superhydrophilicity by adjusting the relative proportion of acetone and water. After solvothermal reduction of the mixed dispersion of graphene oxide and PVDF, Li et al. [44] prepared a superhydrophobic and superoleophilic graphene/polymer aerogels with high absorption capacity for oils and organic solvents. By taking advantages of the low polarizability of perfluorinated carbons and the intrinsic conductive nature of graphene nanoribbons, Wang et al. [45] also prepared a perfluorododecylated graphene nanoribbon film with excellent anti-icing and deicing properties. In Li’s study [46], graphene oxide was firstly treated with spark plasma sintering (SPS) and then reduced at 1050 °C for 1 min, the obtained SPS-reduced graphene oxide demonstrated a WCA of 153° and possessed an impressive bacterial antifouling and inactivation effect against Escherichia coli. From the above literatures, it is obvious that the incorporation of toxic fluorinecontaining chemicals and high-temperature treatment with expensive equipment are usually necessary for the fabrication of graphene-based
Corresponding author at: College of Materials Science and Engineering, South China University of Technology, No 381, Wushan Road, Tianhe District, Guangzhou 510640, China. E-mail addresses: [email protected] (H. Li), [email protected] (X. Zeng).
https://doi.org/10.1016/j.porgcoat.2017.12.001 Received 29 September 2017; Received in revised form 16 November 2017; Accepted 1 December 2017 0300-9440/ © 2017 Elsevier B.V. All rights reserved.
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and dried at 80 °C for 1 h. Next, a predetermined amount of mGO (e.g. the mass ratios of mGO to V-PDMS was varied at 0, 0.1, 0.25 and 0.5, respectively) was added into 10 g of acetone solution containing 0.3 g of V-PDMS, 0.03 g of TMPTA and 0.01 g of Darocur 1173, and sonicated for 30 min to form a homogeneous solution. Subsequently, the dried fabric was dipped into the acetone solution and sonicated for 30 min in the ice water bath to keep the temperature constant, then taken out and exposed under UV light (INTELLI-RAY 400, Uvitron International, Inc., U.S.A.) for 2 min with a distance of 15 cm between the sample and the center of UV light lamp. Thus, the superhydrophobic mGO/PDMS hybrid coating on fabric was obtained.
superhydrophobic materials and surfaces. Recently, our group [47] prepared a thiolated graphene-based superhydrophobic polyurethane sponge with high absorption selectivity and oil/water separation efficiency, and the thiolation process of graphene oxide to thiolated graphene was simple and low-cost. However, the adhesion between thiolated graphene and sponge skeleton still needs to be further strengthened in practical oil/water separation. Herein, we propose a facile approach to fabricating superhydrophobic hybrid coating on polyester fabric for oil/water separation with graphene material. Firstly, graphene oxide (GO) was modified with (3-mercaptopropyl) trimethoxysilane (MPTMS) to achieve hydrophobic mGO with thiol groups. Secondly, the pristine polyester fabric was immersed into the acetone solution containing mGO, vinyl-terminated polydimethylsiloxane (V-PDMS), trimethylolpropane triacrylate (TMPTA) and photoinitiator Darocur 1173, and then exposed under UV light to form crosslinked mGO/PDMS hybrid coating on fabric. The asfabricated coating on polyester fabric exhibited rough morphology and superhydrophobicity. Importantly, the superhydrophobic fabric possessed high oil/water separation efficiency at 99.8% and good reusability. Our method is simple, efficient and cost-effective, and has great potential in the treatment of oil spill accidents and industrial sewage emissions.
2.4. Stability evaluation of superhydrophobic mGO/PDMS hybrid coating on polyester fabric The thermal stability of the superhydrophobic mGO/PDMS hybrid coating on fabric was evaluated by measuring the water contact angle (WCA) after being heated in the range of 30–150 °C with an increasing interval of 30 °C for every 6 h. To comprehensively evaluate the chemical stability, the superhydrophobic fabrics were separately immersed into various solutions including water, toluene, hexane, NaCl solution (1 mol/L), HCl solution (pH = 1) and NaOH solution (pH = 13) for different time, and then rinsed with ethanol and dried at 80 °C for WCA test. In addition, the mechanical stability was also studied via the abrasion test. The 800 mesh sandpaper was acted as the abrasion material, and the superhydrophobic fabric sample was pressed by a weight of 200 g in one direction, and the dragging distance was 15 cm for one cycle.
2. Experiment section 2.1. Materials Graphite powder (325 mesh), sodium nitrate (NaNO3), (3-mercaptopropyl) trimethoxysilane (MPTMS), trimethylolpropane triacrylate (TMPTA, 85%), 2-hydroxy-2-methylproplophenone (Darocur 1173), sodium chloride, hexane, ethanol and oil red O were all provided by Aladdin reagent Co., Ltd., China. Hydrogen peroxide (H2O2, 30%) was bought from Chinasu Specialty Products Co., Ltd., China. Potassium permanganate (KMnO4), sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl, 37%), sodium hydroxide, acetone, toluene, petroleum ether, dichloromethane and trichloromethane were all supplied by Guangzhou Chemical Reagent Factory, China. Vinyl-terminated polydimethylsiloxane (V-PDMS, average MW = 6000) was purchased from Meryer (Shanghai) Chemical Technology Co., Ltd. Methylene blue was obtained from Tianjin Tianxin fine chemical development center (China). Polyester fabric (plain weave fabric, 120 g/m2) was bought from local market. All chemicals were used as received without further purification, and deionized water was used for all the experiments and tests.
2.5. Oil/water separation Oil/water separation test was carried out with a simple homemade filter device composed of two glass tubes and two metal clips. The fabricated superhydrophobic fabric acting as filter membrane was sandwiched between the two glass tubes, and then firmly fixed using metal clips. When oil/water mixture was poured into the upper tube, water was impeded in the tube owing to the water repellency of the fabric, while oil automatically penetrated through the fabric and was collected in the container under the glass tube. Several different kinds of light oil and heavy oil including hexane, toluene, trichloromethane, dichloromethane and petroleum ether were selected for separation test. The oil/water separation efficiency was calculated according to the following Eq. (1):
S eparationefficiency = 2.2. Preparation of MPTMS modified graphene oxide (mGO)
m1 × 100% m0
(1)
where m0 and m1 represented the mass of initial oil and collected oil, respectively.
GO was synthesized from graphite powder according to the modified Hummers’ method [47,48] (see Supporting Information). To achieve the modification of GO, 5 mL of MPTMS and 100 mL of deionized water were firstly added to a 500-mL three-neck flask equipped with a mechanical stirrer and a condenser pipe, and then the pH value was adjusted to 4–5 by adding HCl solution (1 mol/L). After that, 60 mL of GO aqueous solution (3.5 mg/mL) was added under stirring. Subsequently, the mixture was heated to 90 °C and kept for 6 h. Finally, the cooled mixture was filtrated, washed with plenty of deionized water and ethanol separately for three times, then dried at 50 °C for 12 h, and the black mGO was obtained for the next use.
2.6. Characterizations The surface morphologies of the mGO and superhydrophoic fabric were observed on an EVO18 scanning electron microscope (SEM, Carl Zeiss Jena, Germany) at an acceleration voltage of 10.0 kV under high vacuum condition. Fourier transform infrared (FT-IR) spectroscopy was carried out on a Bruker Tensor 27 spectrometer (Bruker Optics, Germany) in the range of 4000–400 cm−1 with a resolution of 4 cm−1 and scanning times of 32, and samples were prepared by the potassium bromide (KBr) tableting method. Chemical compositions of GO and mGO were performed on a X-ray photoelectron spectroscope (XPS, Kratos Axis Ulra DLD, UK) equipped with Al Kα monochromatic X-ray source and three electron take-off angles (30°, 60° and 90°). The surface microstructure and roughness of the pristine and fabricated fabrics were analyzed with atomic force microscopy (AFM, Bruker Multimode 8, USA) in tapping mode with a scanning rate of 0.977 Hz in 3 μm × 3 μm scale. The grafting ratio of MPTMS on GO was
2.3. Fabrication of superhydrophobic mGO/PDMS hybrid coating on polyester fabric The schematic illustration for fabricating superhydrophobic mGO/ PDMS hybrid coating on fabric is presented in Fig. 1. Typically, the pristine polyester fabric with suitable size (25 mm × 25 mm) was separately cleaned with ethanol and deionized water under sonication, 173
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Fig. 1. Schematic illustration for fabricating superhydrophobic mGO/PDMS hybrid coating on fabric.
vibration and alkoxy CeO stretching, respectively [50]. After modification, the intensity of the broad peak at 3392 cm−1 significantly decreased, and a new weak band at 2553 cm−1 corresponding to S-H vibration of MPTMS appeared, confirming the successful grafting of MPTMS on GO [51]. The XPS spectra of GO and mGO are shown in Fig. 3b. Notably, there were only C1s and O1s peaks in the XPS survey spectrum of GO. Differently, the new Si2s, Si2p, S2s and S2p peaks were observed in that of mGO. Additionally, the intensity of O1 s peak significantly decreased and the C1s peak slightly increased, which led to a high C/O atom ratio of mGO at 11.56. The high-resolution C1s corelevel spectra of GO and mGO are displayed in Fig. 3c and d, respectively. It can be seen that the typical XPS C1s spectrum of GO was divided into five peaks at 284.7, 285.6, 286.1, 288.2 and 289.1 ev, attributing to C]C, CeC, CeO, C]O and OeC]O, respectively [51,52]. Comparatively, in the C1s spectrum of mGO, the peak intensity of CeO, C]O and OeC]O dramatically decreased, and the new characteristic peaks of Si-C at 283.7 ev and C-S at 285.9 ev were observed, amply demonstrating that MPTMS had been grafted on GO nanosheets [47,51]. The grafting ratio of MPTMS on GO was also determined by TGA (see Fig. S1) and calculated to be approximately 20.8%.
determined with a TG209 thermogravimetric analyzer (Netzsch, Germany) heating from room temperature to 700 °C at 10 °C/min under nitrogen atmosphere. WCAs were measured with a contact angle meter (DSA100, Germany) equipped with a video capture using about 5 μL of water drops as probe at room temperature, and at least five different positions were selected for measurement to calculate the average value.
3. Results and discussion 3.1. Preparation and structure characterizations of mGO Generally, due to the existence of polar carboxyl, hydroxyl, carbonyl and epoxy groups, hydrophilic GO is not considered to be appropriate for preparing superhydrophobic materials. However, by means of the polar groups especially hydroxyl and carboxyl groups as reactive points, GO can be easily modified to achieve hydrophobicity. In our work, GO was modified with MPTMS to prepare hydrophobic mGO by utilizing the hydrolysis of MPTMS to form −Si-OH groups and the subsequent condensation with hydroxyl groups on GO. The photograph of GO and mGO in water with the same mass concentration of 20% is presented in Fig. 2a. It was notable that GO was well dispersed in water, and the aqueous solution appeared homogeneous brownyellow color. In contrast, the black mGO was unable to disperse in water, and precipitated at the bottom of the bottle. Correspondingly, the WCA of GO was only 31°, and that of mGO reached to 128° as shown in Fig. 2b. The obvious changes in color and WCA can be attributed to the removal of most polar oxygen-containing groups [49]. The SEM images of GO and mGO are shown in Fig. 2c and d, respectively. Clearly, GO exhibited a lamellar structure with some wrinkled and scrolled shapes, and mGO possessed the similar nanosheets while accompanying with some more dense and folded regions. As can be seen from FT-IR spectra in Fig. 3a, the most characteristic peaks of GO located at 3392, 1718, 1622, 1398, and 1082 cm−1 were ascribed to OeH stretching, C]O stretching, C]C stretching, OeH
3.2. Fabrication of superhydrophobic mGO/PDMS hybrid coating on polyester fabric In our work, mGO was not only utilized to construct the roughness of the hybrid coating on the fibers, but also acted as crosslinking points for V-PDMS to form three-dimension crosslinked network structure through the thiol-ene reaction between −SH groups of mGO and terminated eC]C groups of V-PDMS under UV light. Additionally, the residual polar groups on mGO were also beneficial for its adhesion with the polar polyester fibers. By a simple dipping-UV curing method, the superhydrophobic mGO/PDMS hybrid coating on fabric was easily fabricated with mGO, V-PDMS, TMPTA and Darocur 1173. As shown in FT-IR spectrum of the fabric before UV curing (see Fig. S2), the peaks at 174
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Fig. 2. (a) Photograph of GO (left) and mGO (right) in water; (b) WCA images of GO at 31° and mGO at 128°; SEM images of (c) GO and (d) mGO.
1298 cm−1 and 798 cm−1 of Si-C and 1089 cm−1 of Si-O-Si verified the existence of V-PDMS, and the representative peaks locating at 2962 cm−1 and 2900 cm−1 were attributed to the stretching vibrations of −CH3 and −CH2- on mGO and V-PDMS chains [53]. In addition, the stretching vibrations of C]O and OeH assigning to the residual carboxy groups were also observed at 1735 cm−1 and 1404 cm−1, respectively. After UV curing, the weak peaks at 2561 cm−1 of S-H groups of mGO and 1627 cm−1 of eC]C groups of V-PDMS disappeared, demonstrating the successful thiol-ene reaction between −SH groups of mGO and terminated eC]C groups of V-PDMS.
It is well known that the construction of hierarchical micro/nanostructures is crucial for the achievement of superhydrophobic materials. The SEM images and the corresponding WCAs of the superhydrophobic fabrics fabricated with different mass ratios of mGO to VPDMS are illustrated in Fig. 4. Obviously, the pristine polyester fabric showed smooth surfaces and water droplets were gradually absorbed by the hydrophilic fabric (see Fig. S3). The fabric surface only covered with PDMS layer was quite smooth and the WCA was 135° (Fig. 4a). With the mass ratio of mGO to V-PDMS at 0.1, some protrusions were observed on the surface of the fibers and the WCA attained 143° Fig. 3. (a) FT-IR spectra of GO and mGO; (b) XPS survey scans of GO and mGO; (c) C1s XPS spectrum of GO; (d) C1s XPS spectrum of mGO.
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Fig. 4. SEM images of mGO/PDMS hybrid coating on polyester fabrics with different mass ratios of mGO to V-PDMS at (a) 0; (b) 0.1; (c) 0.25 and (d) 0.5, respectively.
interpreted by Cassie-Baxter model [54]. Furthermore, as shown in Fig. 6c, when a continuous jet of water was sprayed on the fabricated fabric, it easily bounced off the surface, further illustrating the excellent water repellency.
(Fig. 4b). When the mass ratios further increasing to 0.25 and 0.5, the hierarchical micro/nano structure of the fiber surface significantly increased, and the WCAs reached to 157° and 153°, respectively (Fig. 4c and d). However, it was worth noting that the WCA of the fabric fabricated with the mass ratio at 0.5 was slightly lower than that at 0.25, which was due to the formation of the enlarged protrusions by the serious agglomeration of mGO nanosheets. Thus, the fabric fabricated with the mass ratio of mGO to V-PDMS at 0.25 was chosen for the following tests. To further investigate the surface roughness of the mGO/PDMS hybrid coating on fabric, AFM was performed and the morphology images of the pristine fabric and the fabricated superhydrophobic fabric are presented in Fig. 5. As shown in Fig. 5a, the surface of the pristine fabric was quite smooth, and the root-mean-square roughness (RMS) was calculated to be only 1.03 nm. In comparison, the superhydrophobic fabric in Fig. 5b obviously exhibited a higher hierarchical roughness with the obvious increase of RMS to 12.4 nm, which was in good consistent with the above SEM and WCA results. Fig. 6a shows the photograph of water and hexane droplets on the pristine fabric and the fabricated fabric, respectively. It was clear that water droplets quickly permeated into the pristine fabric only in several seconds. Differently, on the fabricated fabric, water droplets could keep their sphere shape all the time and even stand on the oil-contaminated area, exhibiting the superhydrophobicity and superoleophilicity. When pressed into water by an external force, the fabricated fabric appeared a mirror-like surface in Fig. 6b, as a result of a uniform air layer existing between the coating layer on fabric and water, which could be
3.3. Stability of superhydrophobic mGO/PDMS hybrid coating on polyester fabric To evaluate the stability, the superhydrophobic fabric was heated subsequently to 30 °C, 60 °C, 90 °C, 120 °C and 150 °C for 6 h, and also separately immersed in water, hexane, toluene, NaCl solution, strong acid and alkali solution for different time to compare the changes of the WCAs. As shown in Fig. 7a, the WCAs of the superhydrophobic fabric had almost no change after heating for 6 h at different temperatures, showing the strong thermal stability. Furthermore, the WCAs of the fabricated fabrics still remained above 150° after being separately immersed into water, hexane, toluene and NaCl solution for 48 h as illustrated in Fig. 7b, and the corresponding photographs of water droplets standing on the treated fabric clearly revealed the good chemical stability of the superhydrophobic fabric (see Fig. S4). Meanwhile, it was notable that the WCAs of the fabricated fabric had no obvious decrease even after being immersed into strong acid solution (pH = 1) for 48 h and alkali solution (pH = 13) for 12 h, respectively. Additionally, the surface morphologies changed little as shown in Fig. 7c and d. The abrasion test was also carried out to evaluate the mechanical stability of the superhydrophobic fabric (see Fig. S5a). After 100 abrasion cycles, the WCA decreased a little, while the water droplets still remained Fig. 5. AFM images of the surface of (a) pristine fabric and (b) superhydrophobic fabric.
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Fig. 6. (a) Photograph of water and hexane droplets on the pristine fabric (left) and the fabricated fabric (right); (b) Photograph of the fabricated fabric immersed into water by an external force; (c) Photograph of a continuous jet of water sprayed on the fabricated fabric.
spherical on the seriously worn-out areas (Fig. S5b). It was mainly attributed to the formation of the crosslinked PDMS network with mGO as crosslinking points.
3.4. Oil/water separation of the superhydrophobic polyester fabric With the increasing industrial sewage and deteriorating marine environment, superhydrophobic material has been considered to be a desirable candidate for oil/water separation. As shown in Fig. 8a and b, Fig. 7. (a) WCAs of the fabricated fabrics heated at different temperatures for 6 h; (b) WCAs of the fabricated fabrics immersed in various solutions; SEM images of the fabricated fabrics after being immersed in (c) HCl solution (pH = 1) for 48 h and (d) NaOH solution (pH = 13) for 12 h.
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Fig. 8. Photographs of the superhydrophobic polyester fabrics selectively absorbing (a) hexane and (b) trichloromethane droplets from water.
separation efficiencies were all above 90% (Fig. 10a). Furthermore, the superhydrophobic fabric exhibited excellent reusability and still remained high separation efficiency at 98.4% after 15 separation cycles (Fig. 10b).
when several dyed droplets of hexane and trichloromethane as the representatives of light oil and heavy oil were dropped into two beakers with water, they quickly spread on water surface and the bottom of the beaker, respectively. Notably, it was very convenient to completely separate the oil from water with the superhydrophobic fabric just in several seconds (see Video S1 and S2). A simple oil/water separation device was set up to further study the massive amount of oil/water mixture. It was composed of two glass tubes, two metal clips and a piece of superhydrophobic polyester fabric. As can be seen from the separation process of oil/water mixture shown in Fig. 9, when trichloromethane/water mixture was poured into the upper glass tube, the red trichloromethane easily penetrated through the fabric and quickly flowed into the container along with the glass tube under the force of gravity, while water was impeded in the upper glass tube owing to the superhydrophobicity of the fabric (see Video S3), and the separation efficiency reached 99.8%. Additionally, several kinds of oils including hexane, toluene, dichloromethane and petroleum ether were also selected as representatives to comprehensively evaluate the oil/water separation ability of the superhydrophobic fabric, and the
4. Conclusions To summarize, we demonstrate a facile dipping-UV curing approach to fabricate superhydrophobic mGO/PDMS hybrid coating on polyester fabric for oil/water separation. With the mass ratio of mGO to V-PDMS at 0.25, the WCA of the fabricated fabric reached 157°, and the corresponding fiber surface appeared micro/nano structure and hierarchical roughness. Due to the formation of the crosslinked PDMS network with mGO as crosslinking points, the superhydrophobic polyester fabric also exhibited excellent thermal stability and chemical stability. Importantly, the fabric could effectively separate oil/water mixture, and possessed high separation efficiency and excellent reusability. The fabrication method is simple, efficient and available for large-scale production, and has the great potential in the treatment of oil spill
Fig. 9. Snapshots for the separation process of trichloromethane/water mixture with the superhydrophobic fabric.
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Fig. 10. (a) Separation efficiencies of the superhydrophobic fabrics for different oil/water mixtures and (b) Separation efficiency of the superhydrophobic fabrics for trichloromethane/water mixture after different separation cycles.
accidents and industrial sewage emissions. Our findings conceivably stand out as a new tool to prepare graphene-based superhydrophobic materials.
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Acknowledgements The work was financially supported by the National Natural Science Foundation of China (51403067) and the Pear River S&T Nova Program of Guangzhou (201710010062).
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Appendix A. Supplementary data [23]
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.porgcoat.2017.12.001.
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